fix(hooks): count HH:MM memory.md rows so the stop reminder can clear

[meketreve]

The bug

The stop hook prints this on every stop, forever, no matter how many summaries have been written:

ACTION REQUIRED: N files were modified this session but no semantic summary was written to memory.md. Append a one-line summary: | HH:MM | description | file(s) | outcome | ~tokens |

countSemanticEntries counts only rows starting with | YYYY-MM-DD, and nothing in the codebase writes rows in that shape:

where	format
src/templates/OPENWOLF.md:41	documents | HH:MM | description | file(s) | outcome | ~tokens |
src/hooks/stop.ts:297	the reminder text asks for | HH:MM | ... |
src/hooks/stop.ts:197	the hook's own rows: `| ${timeShort()} | Session end: ...`
src/hooks/session-start.ts:147	sessions are delimited by ## Session: YYYY-MM-DD HH:MM

So the count is always 0. Following the reminder's own instructions can never satisfy its check — the agent writes a summary, the reminder fires again, and the loop repeats until someone reads the source.

The fix

Decide "today" from the enclosing ## Session: header instead of from the row, then count non-mechanical | HH:MM | rows inside today's block.

Two properties kept deliberately:

• rows with an explicit | YYYY-MM-DD | prefix still count — nothing that already worked regresses;
• rows from an earlier session still don't count — a naive "accept any HH:MM row" would break the check the other way, and the reminder would never fire again once memory.md had a single entry.

The mechanical-row filter is unchanged, so the hook's own Created / Edited / Multi-edited / Session end: / designqc: rows are still excluded.

Tests

New tests/semantic-entries.test.ts, in the style of tests/token-measurement.test.ts. Two cases fail on the old implementation and pass on the new one; the other four pass on both and pin the surrounding behaviour so this can't silently loosen:

case	old	new
HH:MM row inside today's session block	✖ 0	✔ 1
stacked sessions — only today's rows counted	✖ 1	✔ 2
mechanical rows ignored	✔ 0	✔ 0
rows from an earlier session	✔ 0	✔ 0
explicit | YYYY-MM-DD | row	✔ 1	✔ 1
missing memory.md	✔ 0	✔ 0

Verification

Linux (WSL2), Node 24:

• node --test tests/*.test.ts → 32/32 pass
• tsc --noEmit -p tsconfig.hooks.json → clean

Behaviour change is confined to countSemanticEntries; its only caller is checkSemanticSummaries in src/hooks/stop.ts, which is unchanged.